Method For Testing A Batch Of Material Used For Obtaining Layers Of Fibres Intended For Manufacturing A Composite Material Component

ABSTRACT

A method for testing a batch of material used to obtain layers of fibres includes taking a specimen from the batch of material, exerting a tensile force on the specimen, heating the specimen so as to relax the fibres if the specimen contains stresses, after a predetermined period of time, no longer heating and no longer exerting the tensile force on the specimen, measuring a residual extension of the specimen which corresponds to a variation in length of at least a part of the specimen in the direction of the tensile force, before and after the tensile force, and considering the batch of material not to be compliant if the residual extension exceeds a given threshold.

FIELD OF THE INVENTION

The present invention relates to a method for testing a batch ofmaterial used to obtain layers of fibres intended for manufacturing acomposite material component, and also to a method for manufacturing acomposite material component that comprises said test method.

BACKGROUND OF THE INVENTION

In the aerospace domain, composite materials are being used more andmore to reduce the mass of an aircraft. A composite material componentcomprises fibres, for example made of carbon, that are embedded in amatrix of resin, for example thermosetting resin.

In one mode of operation, firstly, a preform of fibres, preferablypre-impregnated fibres, is produced and subsequently polymerized inparticular in an autoclave so as to obtain the final component.

In order to obtain the preform of fibres, layers of fibres are stackedsuccessively one on top of another, the fibres being oriented in atleast one predetermined direction for each layer depending on themechanical characteristics that are desired.

In order to increase productivity, automatic machines such as thosereferred to as ATL (Automated Tape Laying) machines are used to producethe stack of layers of fibres.

According to one embodiment, the layers of fibres are cut out of a stripof fibres stored in the form of a reel, the strip of fibres beingpressed against a support in the form of a film of paper or plasticsmaterial that has the role of a separator when the strip is wound on areel.

The absence of undulations in the fibres is an important criterion forthe quality of the final component.

The undulations in the fibres may originate from the layers of fibres orbe generated during the stacking of the layers or during polymerization.

Thus, prior to being placed in position, the layers of fibres can havestresses which tend to generate the undulations of fibres in the layersof fibres.

For example, during the production of the strip of fibres, when thesupport is in the stretched state at the time when the fibres are placedthereon, the releasing of the extension of the support will bring aboutcompression of the fibres, which will undulate and remain in this statein the preform of fibres.

According to another aspect, with the strip of fibres wound on a reel,there is a difference in tension between the fibres close to theinternal surface and those close to the external surface, resulting inthere being compressed fibres in the region of the internal surface ofthe strip of fibres wound on a reel. This compressed state of the fibresin the region of the internal surface is even greater, the thicker thestrip and the smaller the winding radius. In the case of a strip offibres wound on a reel and close to its expiration date, the resin thatimpregnates the fibres is “drier” such that the fibres close to theinternal surface tend to retain their undulations during the stacking ofthe layers of fibres.

There are no non-destructive tests that ensure the absence ofundulations in fibres. The only test that is possible consists incutting up a finished component into localized sections in order toobserve the layers of fibres under a microscope.

However, this test phase is only possible at the end of the method formanufacturing the components. Consequently, if the layer of fibres has adefect prior to stacking, this is only detected at the end of the methodand the steps of depositing layers and polymerization are carried out invain.

SUMMARY OF THE INVENTION

Aspects of the present invention may remedy the drawbacks of the priorart.

An aspect of the invention is a method for testing a batch of materialused to obtain layers of fibres so as to determine, before it is used toproduce a preform of fibres with the aim of obtaining a compositematerial component, whether the batch of material is compliant and thereis no risk of it bringing about undulations in the composite materialcomponent that is obtained.

According to an aspect of the invention, the test method ischaracterized in that it comprises the following steps that consist in:

taking a specimen comprising fibres from the batch of material,

exerting a tensile force on the specimen,

heating the specimen so as to relax the fibres if the specimen containsstresses,

after a predetermined period of time, no longer heating and no longerexerting the tensile force on the specimen,

measuring a residual extension of the specimen which corresponds to avariation in length of at least a part of the specimen in the directionof the tensile force, before and after the tensile force,

considering the batch of material not to be compliant if the residualextension exceeds a given threshold.

According to one feature, the tensile force is greater than or equal to4 daN/mm² of cross-sectional area of the specimen.

Preferably, the specimen is tautened and kept taut before and after thetensile force during the measurement of the residual extension.Advantageously, a force of around 0.16 daN/mm² of cross-sectional areaof the specimen is applied to tauten the specimen.

According to one mode of operation, the specimen is heated to atemperature of around 100° C.

According to another feature, the specimen is in the form of a striphaving a length of around 10 m, the tensile force being exerted alongthe length of the specimen.

Preferably, the fibres of the specimen are oriented in the direction ofthe tensile force.

According to one mode of operation, the specimen comprises a fixed firstend which is connected to a fixed point of a reference element, and amovable second end at which a force is exerted. In this case, preferablya first mark is provided on the specimen in the vicinity of the secondend and a second mark is provided on the reference element, the firstmark and the second mark being disposed in a direction intersecting thedirection of the tensile force, the residual extension corresponding tothe variation in length before and after the tensile force between thefirst mark and the second mark.

According to another feature, for a specimen having a length of 10 m anda width of around 6 mm, the threshold is around 0.8 mm for a batch ofmaterial having a density of 270 g/m² of pre-impregnated fibres.

According to another feature, for a specimen having a length of 10 m anda width of around 6 mm, the threshold is around 1 mm for a batch ofmaterial having a density of 198 g/m² of pre-impregnated fibres.

According to another feature, for a specimen having a length of 10 m anda width of around 6 mm, the threshold is around 1.2 mm for a batch ofmaterial having a density of 135 g/m² of pre-impregnated fibres.

The invention also relates to a method for manufacturing a compositematerial component, comprising a method for testing a batch of materialused to obtain layers of fibres, prior to the use of said layers offibres.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages will become apparent from the followingdescription of the invention, this description being given purely by wayof example, with reference to the appended drawings

FIGS. 1A-1C illustrate elevational view of a specimen during differentsteps of a method according to an aspect of the invention.

DETAILED DESCRIPTION

According to one mode of operation, a composite material component isobtained by polymerizing a preform of fibres which comprises a stack oflayers of fibres. Preferably, the fibres are pre-impregnated with aresin. By way of example, the fibres are made of carbon and the resin isa thermosetting resin.

The layers of fibres are cut out of a strip of fibres, namely an elementhaving a long length compared with its width.

As an idea of an order of magnitude, the strip of fibres has a thicknessless than 1 mm, a width of around 150 to 300 mm and a length greaterthan or equal to 10 m.

The strip of fibres comprises fibres that are oriented in at least onepredetermined direction depending on the mechanical properties that aredesired for the component.

The strip of fibres is generally stored in the form of a reel.Advantageously, a substrate in the form of a fine film of paper and/orplastics material is pressed against at least one of the faces of thestrip in order to act as a separator when the strip is wound on a reel.

The nature of the fibres and of the resin, the dimensions of the layers,the mode of operation of stacking and the mode of operation ofpolymerization are not described in more detail since they are known toa person skilled in the art.

In any case, the layers of fibres come from a batch of material which isin the form of a reel in one storage mode. Other arrangements may beenvisaged for the batch of material, for example folding in the mannerof an accordion.

Before it is used to form layers of fibres which will then be stackedone on another in order to obtain a preform, the batch of material istested so as to verify whether it comprises stresses that are liable togenerate undulations in fibres in the preform. This solution makes itpossible to avoid the use of a defective batch for manufacturingcomposite material components.

According to one mode of operation, the method for testing the batch ofmaterial used to obtain layers of fibres comprises the steps thatconsist in:

taking a specimen 10 from the batch of material,

exerting a tensile force F on the specimen 10,

heating the specimen 10 so as to relax the fibres if the specimencontains stresses, for example undulations or micro-undulations offibres,

after a predetermined period of time, no longer heating and no longerexerting a tensile force F on the specimen 10,

measuring a residual extension of the specimen 10,

considering the batch of material not to be compliant if the residualextension exceeds a given threshold.

The residual extension corresponds to a variation in length of at leasta part of the specimen 10 in the direction of the tensile force F,before and after the tensile force F has been exerted. It is denoted din FIG. 1C.

The tensile force F should be significant and greater than or equal to 4daN/mm² of cross-sectional area of the specimen, so as to allow thefibres to extend, eliminating the undulations which they may have whenthey are held by the resin at 20° C. Thus, the tensile force F dependson the cross-sectional area of the specimen. For each cross-sectionalarea, the surface of the section of the fibre is extended in a planeperpendicular to the length of the fibre.

Advantageously, the specimen 10 is cut out along the length of the stripof fibres when the batch of material is in the form of a reel with agreat width (150 or 300 mm).

Preferably, the specimen 10 has a length L of around 10 m in order thatthe residual extension is significant and thus easy to measure.

According to one embodiment, the specimen 10 has a length of around 10 mand a width 1 of around 6 to 10 mm.

According to another feature of the invention, the tensile force F isexerted in a direction parallel to the length of the specimen, thelength corresponding to the largest dimension of the specimen.

Advantageously, the specimen 10 is cut out of the batch of material suchthat the fibres of the specimen are oriented in the direction of thetensile force F.

According to one embodiment, the specimen 10 comprises a fixed first end12, connected to a fixed point of a reference element 14 such as atable, and a movable second end 16 at which a force is exerted. Thefirst end 12 is connected to a fixed point so as to ensure the absenceof slip between the first end 12 and the reference element 14.

According to one mode of operation, before the tensile force F isexerted, a first mark 18 is provided on the specimen 10 in the vicinityof the second end 16 and a second mark 20 is provided on a fixedreference element 14, for example the table, the first mark 18 and thesecond mark 20 being disposed in a direction intersecting the directionof the tensile force F and preferably perpendicular to the direction ofthe tensile force F. The first mark 18 and the second mark 20 areadvantageously lines. According to one embodiment, the first mark 18extends from one edge 22 of the specimen and the second mark 20 isdisposed in line with the first mark 18.

According to this mode of operation, in order to determine the residualextension, the distance between the first mark 18 and the second mark 20should be measured when the tensile force F is no longer being exerted.

According to one feature of the invention, during the tensile force F,the specimen is brought to a temperature of around 100° C. According toone embodiment, use is made of a hot-air gun which is moved slowly, forexample at around 100 mm/s, in front of the specimen, along its length,carrying out a number of back-and-forth movements, around ten, such thatthe fibres which potentially have undulations have time to relax underthe effect of the tensile force.

Preferably, the specimen is tautened and kept taut before and after thetensile force F, during the measurement of the residual extension. Tothis end, a force f is exerted at at least one end of the specimen 10,in particular at the second end 16.

The tensile force F is much greater than the force f. “Much greater”means that the tensile force F is at least 20 times greater than theforce f.

As an idea of an order of magnitude, the force f is around 0.16 daN/mm²of cross-sectional area of the specimen. The tensile force F is around 8daN/mm² of cross-sectional area of the specimen.

The provision of the marks 18 and 20 is carried out with the specimen 10taut and the residual extension is carried out with the specimen kepttaut.

The threshold value beyond which the batch of material is no longercompliant and risks bringing about defects in the final component variesin particular depending on the nature of the fibres and the resin, onthe arrangement and density of the fibres, and on the length and thewidth of the specimen.

For a specimen 10 having a length of 10 m and a width of around 6 mm,the threshold is around 0.8 mm for a batch of material having a densityof 270 g/m² of pre-impregnated fibres, around 1 mm for a batch ofmaterial having a density of 198 g/m² of pre-impregnated fibres andaround 1.2 mm for a batch of material having a density of 135 g/m² ofpre-impregnated fibres.

One mode of operation is shown in FIGS. 1A to 1C.

Firstly, as illustrated in FIG. 1A, the first end 12 of the specimen 10is attached to the reference element 14 and the specimen 10 is tautenedby exerting a force f at the second end 16. The marks 18 and 20 areprovided on the specimen 10 and the reference element 14, respectively.

Next, as illustrated in FIG. 1B, a tensile force F is exerted and thespecimen 10 is heated.

After a period of around 15 minutes, heating of the specimen 10 isstopped and the tensile force F is released down to the force f. Onreturning to the force f, the distance between the two marks 18 and 20is measured, as illustrated in FIG. 1C. If this distance, whichcorresponds to the residual extension d, exceeds a given threshold, thebatch of material is declared non-compliant and is not used to producecomposite material components.

1. A method for testing a batch of material used to obtain layers offibres, the method comprising: taking a specimen comprising fibres fromthe batch of material; exerting a tensile force on the specimen; heatingthe specimen; after a predetermined period of time, no longer heatingand no longer exerting the tensile force on the specimen; and measuringa residual extension of the specimen which corresponds to a variation inlength of at least a part of the specimen in the direction of thetensile force, before and after the tensile force.
 2. The methodaccording to claim 1, further comprising determining the batch ofmaterial to be non-compliant if the residual extension exceeds a giventhreshold.
 3. The method according to claim 1, wherein the tensile forceis greater than or equal to 4 daN/mm² of cross-sectional area of thespecimen.
 4. The method according to claim 1, wherein the specimen istautened and kept taut before and after the tensile force during themeasurement of the residual extension.
 5. The method according to claim4, wherein a force of around 0.16 daN/mm² of cross-sectional area of thespecimen is applied to tauten the specimen.
 6. The method according toclaim 1, wherein the specimen is heated to a temperature of around 100°C.
 7. The method according to claim 1, wherein the specimen is in theform of a strip having a length of around 10 m, the tensile force beingexerted along the length of the specimen.
 8. The method according toclaim 1, wherein the fibres of the specimen are oriented in thedirection of the tensile force.
 9. The method according to claim 1,wherein the specimen comprises a fixed first end connected to a fixedpoint of a reference element, and a movable second end at which a forceis exerted.
 10. The method according to claim 9, wherein a first mark isprovided on the specimen in the vicinity of the second end and a secondmark is provided on the reference element, the first mark and the secondmark being disposed in a direction intersecting the direction of thetensile force, the residual extension corresponding to the variation inlength before and after the tensile force between the first mark and thesecond mark.
 11. The method according to claim 1, wherein, for aspecimen having a length of 10 m and a width of around 6 mm, thethreshold is around 0.8 mm for a batch of material having a density of270 g/m² of pre-impregnated fibres.
 12. The method according to claim 1,wherein, for a specimen having a length of 10 m and a width of around 6mm, the threshold is around 1 mm for a batch of material having adensity of 198 g/m² of pre-impregnated fibres.
 13. The method accordingto claim 1, wherein, for a specimen having a length of 10 m and a widthof around 6 mm, the threshold is around 1.2 mm for a batch of materialhaving a density of 135 g/m² of pre-impregnated fibres.
 14. A method formanufacturing a composite material component, comprising a method fortesting a batch of material of layers of fibres prior to the use of saidlayers of fibres, the method for testing comprising: taking a specimencomprising fibres from the batch of material; exerting a tensile forceon the specimen; heating the specimen so as to relax the fibres if thespecimen contains stresses; after a predetermined period of time, nolonger heating and no longer exerting the tensile force on the specimen;and measuring a residual extension of the specimen which corresponds toa variation in length of at least a part of the specimen in thedirection of the tensile force, before and after the tensile force.